This invention is directed to electromagnetic force machines and, more particularly, to electromagnetic force machines suitable for performing non-destructive tests on or removing dents from a panel.
In the past, a variety of electromagnetic force (EMF) machines have been developed for use in the production and maintenance of panels to perform non-destructive tests on panel bonds and to remove dents. U.S. Pat. No. 4,148,091 issued to Karl A. Hansen et al on Apr. 3, 1979 entitled xe2x80x9cElectromagnetic force machine with universal portable power supply,xe2x80x9d and U.S. Pat. No. 3,825,819 issued to Karl A. Hansen et al on Jul. 23, 1974 entitled xe2x80x9cDynamic Proof Loading of Metal Bond Structures Using Pulsed Magnetic Fieldsxe2x80x9d describe such a machine. U.S. Pat. No. 5,046,345 issued to Peter B. Zieve on Sep. 10, 1991 entitled xe2x80x9cPower Supply for Electromagnetic Proof Load Tester and Dent Removerxe2x80x9d teaches the power supply for such dent pullers.
Aircraft mechanics commonly use electromagnetic energy to produce a pulling (tension) force in electrically conducting members such as aircraft metal skin panels. Controlling the current through a working-coil generates magnetic fields in metal skin panels. A magnetic field with the capability of drawing the dent from a work piece must extend across a gap between a working-coil through the adjacent work piece and extending behind. The field induces a current and a corresponding magnetic field within the conductive work piece. Rapid changes in the magnetic field impart forces on induced current within the work piece. The pulling force is local and works to repair dents. The current in the coil is the result of a series of charge pulses under voltage. The discharge of capacitors creates these pulses
To set the stage for dent pulling, chargers charge both fast- and slow-banks of capacitors. When each bank is fully charged, a trigger closes an SCR allowing the slow-bank capacitors to discharge a large current at a relatively low voltage (less than 1000 volts). The discharged current flows through a blocking inductor that smoothes the pulse in a manner to produce a slow rising current pulse. This pulse continues through a work coil positioned adjacent the dented area of the panel.
Current in the working-coils produces corresponding magnetic fields. The current through the work coil creates a magnetic field in and immediately behind the work piece, and in the gap between the coil and the work piece. When the slow current pulse has reached its peak amplitude, approximately 2 milliseconds from initial discharge, a timing pulse to a spark-gap discharges the fast-capacitor bank. The spark-gap releases a current pulse at a high voltage (approximately 2-15 kV) in the opposite direction through the working-coil. The fast-capacitor bank fires on the blocking inductor and working-coil in parallel to produce a fast rising current pulse opposite in polarity to the slow pulse.
The fast-capacitor bank discharge decreases the current through the coil. The current flow in the working-coil quickly diminishes the current in about 10-30 xcexcSec. (Times reflect a normal application and not the functional range.) The magnetic field in the gap between the coil and the work piece quickly is diminished. The difference in the magnetic fields in and behind the work piece as compared to that in the gap produces a Lorentz force. That force pushes the depressed, dented, area outward.
The timing of the fast-capacitor bank discharge in relation to the slow-capacitor bank discharge determines the result. The interplay of the pulses creates a precise set of magnetic field states within the work piece resulting in the force that pulls the dent. That interplay of pulses depends upon the unfailing performance of the gas-filled spark-gap.
Gas-filled spark-gaps do fail. The hold-off voltage of the device falls very slowly during life until it reaches a critical point from which it falls very rapidly; this is due to the deposit of a metallic coating on the internal insulator surface separating the electrodes. High voltage, high current sparking heats conduction spots on the surface of the electrodes. The metallic coating is re-deposited metal, vaporized and eroded from the electrodes.
The lifetime of a gas-filled spark gap is a function of the number of firings, the charge transfer per firing, and the peak current through the spark gap on each firing. The gas-filled spark-gaps tend to fail in two catastrophic modes, leading to, alternately, prefires and misfires. Prefires may occur without a triggering pulse. A prefire discharge occurs when the applied DC voltage is greater than the resulting hold-off voltage caused by the deterioration of the spark gap. A misfire, on the other hand, is a failure of the tube to break down with the triggering pulse.
A misfire results in non-performance by the electronic dent removing apparatus; a prefire, the application of a rapid unplanned tension force on the work piece without the countervailing slow pulse magnetic field. The prefire of the fast-capacitor bank yields a very sharp pulse that dents the work piece. The leverage of the machine to create a dent is greater than its ability to pull them out. The prefire causes a very severe dent and often results in the need for the removal and replacement of the skin panel.
Because of the sudden, unpredictable, and catastrophic results from failure of gas-filled spark-gaps in electronic dent remover power supplies, there is an unmet need in the art for a means to predictably discharge the fast-bank capacitors. Such predictable discharges would remove the risk of the catastrophic effects of prefire discharge of the fast-bank capacitors through the coil.
The present invention eliminates the several dangers presented by the use of gas-filled triggered spark-gaps to discharge the fast-capacitor bank in an electronic dent remover. The enhanced performance range of the vacuum triggered spark-gap in conjunction with dump circuits across the capacitor banks provides greater safety, reliability, and predictability in operation.
The invention is a power supply for an electromagnetic force system, operating by alternately energizing a working coil with a slow current pulse from a bank of capacitors through a blocking inductor, each capacitor bank connected in parallel with a dumping circuit, for discharging said slow current pulse system through the blocking inductor at a selected time; and then an opposite fast current pulse system, resulting in a large field gradient across the work piece thereby creating a pulse of tension force on the work piece. The clamping circuit across the blocking inductor provides an alternate path for discharging the energy from the blocking inductor, further enhancing the performance of the electronic dent remover.